1. Field of the Invention
The invention relates in general to power supply circuit for lamp and more particularly to the power supply circuit and its transformer for driving a plurality of lamps and balancing the currents of the lamps.
2. Description of the Related Art
The brightness of the LCD, i.e. LCD TV, has to increase while the screen size thereof is increasing. The backlight module of the LCD provides the higher brightness by increasing the number and size of the lamps, which are cold-cathode fluorescent lamps for example.
FIG. 1 is a diagram of conventional power supply circuit of a plurality of lamps. The power supply circuit 100 is used in a backlight module that provides the light source for LCD. The power supply circuit 100 includes a DC-AC converter 102, a transformer 104 for driving lamps L1 and L2. DC-AC converter provides an alternative voltage to the transformer 104, which in turn boosts up the alternative voltage for driving lamps L1 and L2. The transformer 104 includes a primary coil set 106 and a secondary coil 108.
The currents respectively flowing through the lamps L1 and L2 may not be the same due to the slightly different impedances, which are caused by the different wire impedances and the different lamp characteristics, and thus the brightness of the lamps L1 and L2 may not be the same, such that the image quality of the LCD and the longevity of the lamps would be affected. Therefore, the issue of unbalanced currents while driving multiple lamps is to be solved.